Many forms of electric arc welding with a consumable electrode use welding guns which have a contact tip, sometimes referred to as a contact tube or electrical contact tube or tip. These forms of welding include gas metal arc welding (GMAW), sometimes referred to as metal inert gas (MIG) welding, as well as submerged arc welding (SAW) and flux cored arc welding (FCAW).
The contact tip is a critical element in welding guns for electric arc welding with a consumable electrode. Its main function is to enable electric current from a welding power supply to be continuously transported to a wire or strip comprising the consumable electrode. Contact tips are made of metal, almost exclusively of copper or a copper alloy, because of the high electrical and thermal conductivity of these metals. Usually contact tips are of hard drawn copper of high purity, or of an alloy such as Cu-2% Be, Cu-0.5% Be and suitable Cu—Cr—Zr alloys.
There is extensive prior art in relation to arc welding equipment, including contact tips. Examples include journal articles such as:
“Fluctuations of the Wire Feed Rate in Gas Metal Arc Welding” by Yamada et al, Welding Journal, September 1987, pp. 35 to 42;
“Understanding Contact Tips Longevity for Gas Metal Arc Welding” by Villafuerte, Welding Journal, December 1999, pp. 29 to 35;
“The Physics of Welding” by J. F. Lancaster, 2nd Ed., Permagen Press, 1986;
“Advanced Welding Processes” by J. Norrish, IOP Publishing Ltd., 1992; and
“Heat Effects of Welding” by D. Radaj, Springer Verlag, 1992.
Further examples are provided by patent literature including GB-2074069 to Folke et al (ESAB Limited); GB-2170133 to Cooke (R. E. Cooke & Sons (Burton) Ltd.; DE 4006138 by Lange; WO98/12011 by Davis; and the following United States patent specifications:
1233434 to Zuck2289938 to Smith2379470 to Baird2428849 to Kratz et al2666832 to Landis et al2679571 to Chappel2735920 to Valliere2754395 to Scheller et al2761049 to McElrath et al2778910 to Landis et al2810063 to Brashear Jr.2866079 to Morley et al2903567 to Piekarski et al2957101 to Barkley2965746 to Cresswell3025387 to Kinney3089022 to Kinney3103576 to Miller3309491 to Jacobs3366774 to Nuss et al3469070 to Bernard et al3470349 to Sievers3488468 to Carbone3514570 to Bernard et al3529128 to Cruz Jr.3536888 to Borneman3576423 to Bernard et al3585352 to Zvanut3590212 to Corrigall et al3596049 to Ogden3597576 to Bernard et al3617688 to Fogelstrom3676640 to Bernard et al3697721 to Robba et al3716902 to Pearce3783233 to dal Molin3825719 to Jonsson3878354 to Frantzreb Sr.4258242 to Fujimori et al4309590 to Stol4361747 to Torrani4560858 to Manning4575612 to Prunier4672163 to Matsui et al4937428 to Yoshinaka et al4947024 to Anderson5101093 to Matsui et al5192852 to Pike5278392 to Takacs5288972 to Wujek5352523 to Zurecki et al5556562 to Sorenson5618456 to Kim5635091 to Hori et al5721417 to Craig5726420 to Lajoie6093907 to Hidaka6130407 to Villafuete6429406 to Sattler.
The principal issues affecting productivity in GMAW are heat input and deposition rate (hence wire melting rate). Heat input is proportional to welding voltage and current and inversely proportional to the travel speed of the welding torch relative to a work piece being welded. The welding torch may be held stationary and the work piece moved relative to the torch, the work piece may be held stationary and the torch moved relative to the work piece, or each of the torch and work piece may move but with relative movement therebetween. Heat input affects weld penetration, cooling time, weld distortion and metallurgical properties in deposited weld metal and/or in adjacent zones of the work piece. In general, it is advantageous to minimise heat input for a given deposition rate.
For a given contact tip of a welding torch, it is possible to establish a theoretical upper limit for the melting rate for a wire used as a consumable electrode, and also a theoretical lower limit for the melting rate of the wire. The principles can be detailed by reference to an established formula for wire deposition rate for GMAW. The above-mentioned text by Norrish presents a formula for wire melting rate. An equivalent algebraic representation may be expressed by the formula:W=aI+bLI2 where:    “W” is the wire feed rate (usually expressed in metres per minute),    “I” is the welding current in amperes,    “a” is a coefficient representing heating of the wire by the welding arc,    “b” is a coefficient for resistive heating of the wire, and    “L” is the relevant length of the wire subjected to resistive heating.
The parameters “a” and “b” depend on the wire diameter (in the case of wire of circular cross-section, or equivalent diameter for wire of other cross-sections) and also on the wire composition. These parameters may be derived for each consumable from measured or published data for deposition rate or wire feed speed.
For simplicity, but without loss of generality in use of the above-indicated formula, welding in the down hand position is assumed. There are essentially two mechanisms responsible for melting wire consumables in GMAW. The first is heating of the wire by the electric arc established between the end of the wire and the surface top of a weldpool. In the formula, this arc-related mechanism is represented by the term “aI”. The second mechanism is resistive heating of the wire by the current established in the wire after the wire makes electrical contact with the contact tip, and this resistive-related mechanism is represented by the term “bLI2”.
The parameter “L” in the above-indicated formula represents the length of the wire between the effective contact point of the wire in the contact tip and the top of the welding arc. This length of the wire differs from the normal use of the parameter L, in which that length is taken as the length of wire exposed beyond the outlet end of the contact tip to the top of the welding arc.
In relation to the resistive heating, the interpretation of L in normal use, i.e. the electrode extension or stick-out, cannot be relied on. It can be appropriate where the wire makes electrical contact with the contact tip at the outlet end of the bore of the contact tip. In such case, the interpretation corresponds to that for the above-indicated formula, at least where there is a single contact point between the wire and the contact tip. However, where there is a single contact point, this can be at any location along the contact tip bore, from the inlet end to the outlet end of the bore. Also, the location can vary between those extremes during a welding operation, and further variation can result from there intermittently being at least two contact points along the length of the bore. It is usual for the length of the bore to be greater than the electrode extension or stick-out and, as a consequence, there can be variability of in excess of 100% in the actual length of the wire subjected to resistive heating; both between successive welding operations and during a given welding operation. That is, there can be variability in excess of 100% in the value of L for the purpose of the above-indicated formula as compared with a measure of L to determine electrode extension or stick-out.
Where the actual length of the wire subjected to resistive heating varies, whether between successive welding operations or during a single welding operation, there can be a substantial variation, in the required welding current at a given wire feed rate. As a result, instantaneous heat input can vary substantially, with adverse consequences for welding performance.
The principles as described above for wire melting rate apply to GMAW carried out with welding power supplies operating under essentially constant voltage conditions. In order to control the mode of droplet transfer by electronic means, pulsed power supplies are also used for GMAW. Pulses of electric current are applied to heat the wire consumable and to induce droplet detachment. The relation between wire melting rate and current is more complex than the relation given in the equation presented previously. Nevertheless there is a strong dependence of wire melting rate on preheat length and it is critical to performance to maintain a continuous current delivery area within the tip.
The second issue related to the performance of a tip is the feedability of the wire through the tip. The reproducibility with time of the processes associated with droplet transfer process requires that a uniform wire feed speed be established and maintained. One of the objectives of this invention is to enable reliable welds to be deposited at wire feed speeds substantially in excess of those possible with conventional GMAW. The feed force must therefore be as low as practicable so that the mechanical work applied to the tip is reduced. Mechanical work results in wear. Wear creates problems in the electrical contact area and ultimately leads to tip failure and defects in the weld.